Purifying magnesium



Patented July 26, 1938 UNITED STATES PATENT OFFICE rumrxnvc MAGNESIUM No Drawing.

9 Claims.

The present invention relates to improvements in purifying magnesium or its alloys.

An object of the invention is to provide a method which permits the obtainment of a substantial- 5 1y perfect refinement and minimum loss of the metallic material to be refined.

A further object is to develop a method for the purification of metal initially tree from chlorine, which enables other impurities to be eliminated without chlorides becoming entrained in the refined material.

For the refining of magnesium or its alloys there are commonly employed salt melts which float as a coating on the molten material or which are brought into intimate contact with this material in some way or other (for example by stirring in). The refining agents that have proved most satisfactory consist of chlorides or sub stances containing chlorine compounds. The employment of these refining methods wherein a bath of a fluid flux is employed, is found to be attended by the drawback that residual traces of chlorides are left behind in the refined material. In spite of the efiorts made in the art for years past to improve this type of refining method it has not proved possible to eliminate completely this troublesome phenomenon. When the object in view is to remove from metallic material which is initially free from chlorine, slight proportions of other impurities of a non-volatile nature, it amounts to a very serious drawback if the material takes up even traces of chlorides in the course of the refining treatment.

The present invention overcomes this drawback by intimately mixing the molten metallic material with a refining agent substantially consisting of fluorine compound which, at the melting or casting temperature, becomes vaporized or decomposed with the liberation of at least one gaseous or vaporousconstituent. Thus in contrast to the known methods there are employed salts which are not fused, in an undecomposed condition, at the working temperature, the desired purifying eflect being achieved in the presout process by the gases or vapors evolved from the fluorine compounds within the molten material to be refined. In this manner the elimination of the impurities present is brought about without any undesirable residue being retained by the material under treatment.

A group or chlorine-free fluorine compounds that are particularly well suited for the carrying out of the present method are the fluosilicates (metallic fluosilicates) The metal salts oi hydrosllicofiuoric acid (HSlFo) on being heated, give Application June 12, 1936, Serial No. 84,909. In Austria August 2, 1935 off gaseous silicon tetrafluoride (SiFA) which becomes uniformly finely distributed in the metallic melt while metallic fluoride remains behind in the non-fused state and becomes spread over the melt in the form of a coating. Magnesium fluosilicate (MgSiFe) has proved to be best suited for the purpose in view, but sodium and aluminium fluosilicate (NazSiFa, AlSiF-n or heavy metal salts of hydrosilicofiuoric acid for example are also suitable for use in the present method.

Next in order of utility for the present purpose come the fiuoborates. Furthermore, similar refining eilects can also be obtained with neutral or acid ammonium fluoride or salts containing ammonium fluoride chemically combined in a complex form (such as for example ammonium silicofluoride ((NH4)2SiFs)), or with mixtures of ammonium fluorides. Neutral ammonium fiuoride (NHiF) vaporizes already at temperatures far below the melting point of magnesium, while the acid salt (NH4F.HF) volatilizes in the heat in the form of white vapor; in both cases the evolved vapors make their way eddyingly through the melt. Ammonium silicofluoride gives up gaseous silicon tetrafiuoride at the working temperature, and at the same time yields ammonium fluoride vapors. Since with the ammonium salts the liberation of gas takes place very vigorously it is advisable to use these salts by themselves only for the refining oi small-sized material with which they can be thoroughly intermixed before charging; only the metallic fluosilicates are well suited for use by themselves in the refining of large-sized material or material containing a large proportion of slag. For the treatment of all these materials there can be used a mixture of ammonium fluoride with metallic fluoborate or with metallic fiuosilicate, the last-mentioned mixtures being particularly advantageous for this purpose.

If the material to be refined is in large pieces, for example blocks or cast metal scrap, the full success of the present method depends on the taking of precautions to ensure that the salts used become vaporized or decomposed only within the melt to be refined. To this end the refining agent may for example be packed, with as complete exclusion of air as possible, in metal tubes (aluminiumor magnesium tubes), and introduced into the melt in this form, for example by rapid immersion.

In refining fluxes, the fluorides of magnesium, calcium, aluminium, and other metals have been used for thickening the magnesium chloride melt. Moreover, it has also been proposed to use mixtures of alkali fluorides and magnesium chloride (MgCla) as refining melts. Efforts to improve the magnesium chloride melt have also been made in another direction by the employment of magnesium chloride (MgFn) in place or magnesium chloride together with the addition of a slight quantity of metallic calcium. All these fluxes, however, belong to the group of the so-called "chemical fluxes which act chemically in a molten state, no substantial decomposition of the flouride used taking place at theworking temperatures. The use of such fluorine containing chemical fluxes is attended by the inconvenience that, owing to the higher melting points of fluorides, higher working temperatures must be employed.

Ammonium fluorides and metallic silicofluorides are employed in the casting of magnesium and magnesium alloys in water bound molds, and more particularly green sand molds, to protect the cast metal from the influence of the air and from the attack of the water vapor generated in the mold. Moreover, it has been proposed to provide an atmosphere containing either elemental fluorine or the vapors of a compound of fluorine in contact with the exposed surface 01' the melt of a readily oxidizable metal (such as magnesium or alloys thereof) with the view of inhibiting the oxidation of the said metallic material while maintained in the molten condition. Similarly it has been proposed, in conjunction with substituting calcium chloride (CaClz) and sodium chloride (NaCl) fluxes for the previously used magnesium chloride (MgCiz) fluxes, to agitate the molten metal with the flux in the presence of a fluorine-containing protective atmosphere. It is a peculiarity of magnesium chloride fluxes that they protect the exposed surfaces of the melt from contact with the atmosphere by forming a persistent film thereon which prevents excessive oxidation of the molten magnesium. n the other hand, the action of calcium chloride-sodium chloride fluxes which do not form such a surface film, is limited largely to one of purification. The addition of a relatively small amount of a volatilizable solid compound of fluorine or of gaseous fluorine-containing compounds, therefore, has been provided to produce a protective atmosphere over the metal.

In contrast to this known process in which it is a molten flux of non-voiatilizable chlorides that brings about the refining action, it is the crux of the present invention that the refining agent itself substantially consists of fluorine compounds which, at the melting or casting temperature, become vaporized or decomposed with the liberation of at least one gaseous or vaporous constituent. That fluorine compounds could be used with advantage as refining agents for magnesium and its alloys in lieu of known salt melts, such as for instance magnesium chloride or calcium chloride-sodium chloride fluxes, has not hitherto been recognized.

Examples (1) Granules of pure magnesium contaminated with carbon and magnesium oxide (MgO), as yielded by the distillation of electrothermically obtained magnesium, are intimately mixed with 1% of their weight of magnesium fiuosilicate (MgSiFc), and heated to melting in a closed iron crucible. In the course of the melting process the magnesium fluosilicate becomes decomposed to silicon tetrafluoride (Sin) and magnesium fluoride (MgFa). The gaseous silicon fluoride bubbles vaporous or gaseous constituent, while through the melt and trees it of the above-mentioned impurities: at the same time the disengaged magnesium fluoride rises to the surface and spreads out on top of the melt.

(2) For refining ingots of pure magnesium there is employed a mixture comprising 90% of magnesium fluosilicate (MgSiFs) and of ammonium fluosilicate ((NHOaSiFs) in a quantity amounting to 1% by weight of the metal to be refined. This mixture is packed as tightly as possible in a magnesium tube. The ingots are heated in a closed iron crucible to a temperature close above the melting point whereupon the magnesium tube is immersed in the heated metal bath while at the same time the melt is thoroughly agitated. The gases generated by decomposition of the salts bubble through the molten metal and free it completely of non-metallic included matter such as nitrides and oxides, and also of slags.

(3) In the same manner as described in Example 2 a mixture comprising 90% of magnesium fiuosilicate (MgSiFa) and 10% of ammonium fluoride (NHiF) can be used for example for the refining of commercial magnesium alloys, in which case it is advisable to work with larger quantities of the refining agent (up to three percent by weight of the metal to be refined). The mixture can be pressed into an aluminium tube.

(4) For reflningscrap of commercial magnesium alloys there is employed a mixture consisting of 60% of magnesium fluosilicate (MgSiFs), of sodium fluosilicate (NazSiFs) 10% of ammonium fiuosilicate ((NHOzSiFs), 5% of ammonium fluoride (NH4F), and 5% of ammonium bifluoride (NHiFl-lF), in a proportion of two percent by weight of the metal to be refined, closely packed in a magnesium or aluminium tube.

Throughout the specification and in the appended claims the term fluorine containing vaporous or gaseous constituent is used to include vapor or gas containing fluorine either in its elemental or in combined form.

The term "melting and casting temperature whenever used in the specification and in the following claims means a temperature above the melting point of magnesium and alloys rich in magnesium in the range normally encountered in the foundry operations of melting, purifying and casting magnesium and the said alloys, 1. e., a range between 600 and 900 C.

The term magnesium is used in the following claims to include not only magnesium but also.

alloys thereof wherein the magnesium content predominates. Moreover, as used in the claims, the term "ammonium fluoride means not only neutral or acid ammonium fluoride but also ammonium salts the anion of which contains fluorine chemically combined in a complex form, examples being ammonium fluosilicate and ammonium fluoborate, and mixtures of any salts of this kind.

What I claim is:

1. The method of purifying magnesium, which comprises intimately mixing the molten metallic material with a solid practically chlorine-free refining agent substantially consisting of fluorine compound which, at the melting or casting temperature, becomes vaporized or decomposed with the liberation of at least one fluorine-containing the said fluorine compound in an amouit suiflm; .t to effect the required purifying action as the refining agent proper.

2 The th d of urifying magnesium comprising distributing within a... molten metal fluorine in combined form to cause fluorine-containing vapor to disengage, by vaporization or decomposition, throughout the molten metal, in a quantity sufllcient for effecting the required purifying action without the assistance of chloride.

3. The method of purifying magnesium, which comprises intimately mixing the molten metallic material with a solid refining agent substantially consisting of fluorine compound which, at the melting or casting temperature, becomes vaporized or decomposed with the liberation of at least one fluorine containing vaporous or gaseous constituent, while introducing the refining agent into the batch of molten material in a closely packed state, to ensure that the fluorine compound becomes vaporized or decomposed only within the melt.

4. The method of purifying magnesium, which comprises intimately mixing the molten metallic material with a solid refining agent selected from the group consisting of metallic fluosilicate, metallic fluoborate and ammonium fluoride, in an amount sufficient to efl'ect the required purifying action, by a vaporization product or gaseous decomposition product liberated at the temperature prevailing in the melt without the assistance of chloride.

5. The method of purifying magnesium, which comprises intimately mixing the molten metallic material with a mixture of metallic fluosilicate and ammonium fluoride in a total amount which suflices for substantially freeing the molten magnesium, by a vaporization product or gaseous decomposition product liberated at the temperature prevailing in the melt, from non-metallic impurities.

B. The method of purifying magnesium, which comprises intimately mixing the molten metallic material with a mixture of magnesium fluosilicate and ammonium fluoride in a total amount which 'suflices for substantially freeing the molten magnesium, by a vaporization product or gaseous decomposition product liberated at the temperature prevailing in the melt, from non-metallic impurities.

'7. The method of purifying magnesium, which comprises intimately mixing the molten metallic material with a mixture of magnesium fluosilicate, sodium fiuosilicate and ammonium fluoride in a total amount which suffices for substantially freeing the molten magnesium, by a vaporization product or gaseous decomposition product liberated at the temperature prevailing in the melt, from non-metallic impurities.

8. The method of purifying magnesium, which comprises intimately mixing the molten metallic material with a mixture of metallic iiuoborate and ammonium fluoride in a total amount which suflices for substantially freeing the molten magnesium, by a vaporization product or gaseous decomposition product liberated at the temperature prevailing in the melt, from non-metallic impurities.

9. The method of purifying magnesium, which comprises intimately mixing the molten metallic material with a mixture of magnesium fluoborate and ammonium fluoride in a total amount which suflices for substantially freeing the molten magnesium, by a vaporization product or gaseous decomposition product liberated at the temperature prevailing in the melt, from non-metallic impurities.

GEORG SCHICHTEL.

CERTIFICATE OF CORRECTION.

Patent No. 2, 121;,957

July 26, 1953.

GEORG SCHICHTEL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, first column, line 55, for "(HSiF read (H SiF page 2, first column, line 5, for the word "chloride" read fluoride; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

I Signed and sealed this 20th day of September, A. D. 1958.

(Seal) Henry Van Arsdale Acting Commissioner of Patents.

ing vapor to disengage, by vaporization or decomposition, throughout the molten metal, in a quantity sufllcient for effecting the required purifying action without the assistance of chloride.

3. The method of purifying magnesium, which comprises intimately mixing the molten metallic material with a solid refining agent substantially consisting of fluorine compound which, at the melting or casting temperature, becomes vaporized or decomposed with the liberation of at least one fluorine containing vaporous or gaseous constituent, while introducing the refining agent into the batch of molten material in a closely packed state, to ensure that the fluorine compound becomes vaporized or decomposed only within the melt.

4. The method of purifying magnesium, which comprises intimately mixing the molten metallic material with a solid refining agent selected from the group consisting of metallic fluosilicate, metallic fluoborate and ammonium fluoride, in an amount sufficient to efl'ect the required purifying action, by a vaporization product or gaseous decomposition product liberated at the temperature prevailing in the melt without the assistance of chloride.

5. The method of purifying magnesium, which comprises intimately mixing the molten metallic material with a mixture of metallic fluosilicate and ammonium fluoride in a total amount which suflices for substantially freeing the molten magnesium, by a vaporization product or gaseous decomposition product liberated at the temperature prevailing in the melt, from non-metallic impurities.

B. The method of purifying magnesium, which comprises intimately mixing the molten metallic material with a mixture of magnesium fluosilicate and ammonium fluoride in a total amount which 'suflices for substantially freeing the molten magnesium, by a vaporization product or gaseous decomposition product liberated at the temperature prevailing in the melt, from non-metallic impurities.

'7. The method of purifying magnesium, which comprises intimately mixing the molten metallic material with a mixture of magnesium fluosilicate, sodium fiuosilicate and ammonium fluoride in a total amount which suffices for substantially freeing the molten magnesium, by a vaporization product or gaseous decomposition product liberated at the temperature prevailing in the melt, from non-metallic impurities.

8. The method of purifying magnesium, which comprises intimately mixing the molten metallic material with a mixture of metallic iiuoborate and ammonium fluoride in a total amount which suflices for substantially freeing the molten magnesium, by a vaporization product or gaseous decomposition product liberated at the temperature prevailing in the melt, from non-metallic impurities.

9. The method of purifying magnesium, which comprises intimately mixing the molten metallic material with a mixture of magnesium fluoborate and ammonium fluoride in a total amount which suflices for substantially freeing the molten magnesium, by a vaporization product or gaseous decomposition product liberated at the temperature prevailing in the melt, from non-metallic impurities.

GEORG SCHICHTEL.

CERTIFICATE OF CORRECTION.

Patent No. 2, 121;,957

July 26, 1953.

GEORG SCHICHTEL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, first column, line 55, for "(HSiF read (H SiF page 2, first column, line 5, for the word "chloride" read fluoride; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

I Signed and sealed this 20th day of September, A. D. 1958.

(Seal) Henry Van Arsdale Acting Commissioner of Patents. 

